Casing and tubing for wells are made up in sections and lowered into the well bore. The sections have been joined by threaded connections with the sections having box and pin ends for direct assembly or with the sections having pin ends joined by an internally threaded coupling. As wells get deeper and encounter corrosive conditions, such as sour gas and salt water, there is a need for higher connection efficiencies in tensile and compressive strength as well as greater resistance to internal and external well bore pressures without substantial increases in the connection diameter.
An early effort to increase the strength of well tubular connections is shown in U.S. Pat. No. 2,772,102 which suggests threads having a normal load bearing face and a tapered flank with the root and crests being in sealing contact at the inner end of the threads in the coupling. U.S. Pat. No. 2,980,451 suggests the use of a seal ring in a groove in the female threads to accomplish the seal. U.S. Pat. No. 3,224,799 discloses a tubular connection in which the load bearing face has a slight negative angle and the other face has an angle in the range from 30.degree. to 60.degree.. U.S. Pat. No. 3,489,437 includes a reverse shoulder at the inner end of the female threads for engaging a similar taper on the pin end so that the pin is urged outward to assure continued engagement of the threads near the end of the pin. U.S. Pat. No. 3,508,771 includes a shoulder on the female member against which the male member engages to protect the threads from damage when the connection is torqued.
An undercut type of threading is shown in U.S. Pat. No. 3,989,284. Two sets of slightly tapered threads in a tubular connection with a seal between the two sets of threads and a reverse shoulder on the female member engaged by the end of the male member are shown in U.S. Pat. No. 4,009,893. A similar connection with a double set of cylindrical threads is shown in U.S. Pat. Nos. 4,161,332, 4,192,533 and 4,244,607.